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Smooth Muscle Antibody

Autoimmune hepatitis is an unresolving inflammation of the liver of unknown etiology that is associated with high concentrations of autoantibodies, pronounced hypergammaglobulinemia and the absence of cholestasis. The definitive diagnosis requires the exclusion of viral, hereditary and drug induced liver disease. Autoimmune hepatitis occurs predominantly in women and affects all ages. It is responsible for up to 18% of chronic hepatitis cases not attributable to viruses or alcohol.

Three types of autoimmune hepatitis have been proposed based on autoantibody associations.

Type

Population Affected

Autoantibody

1

Young- middle aged women

ANA &/or ASMA

2

Western European children

Rare in U.S.

Liver kidney microsomal (LKM) antibody

3

Young women with systemic autoimmune disease

Antibody to Soluble liver antigen

(SLA)

Type 1 autoimmune liver disease is characterized by the presence of high levels of anti-nuclear antibody (ANA) or anti-smooth muscle antibody (ASMA). ASMA are IgG or IgM antibodies that are primarily directed against F-actin. Since F-actin is present in all smooth muscle fibers, these antibodies are not organ specific. Smooth muscle antibodies are found in:

  • 70 to 90% of patients with chronic active hepatitis
  • 70 to 90% of patients with chronic persistent hepatitis
  • 30 to 40% of patients with primary biliary cirrhosis
  • 15% of patients with cryptogenic cirrhosis

Approximately 40% of patients with chronic hepatitis C infection have a positive ANA or ASMA, usually in low titer. Elevated smooth muscle antibody titers have also been reported in a small number of patients with viral hepatitis, infectious mononucleosis, malignant tumors, alcoholic cirrhosis, and 5% of normal patients.

Traditionally, ASMA have been detected with an immunofluorescent assay that looked for antibody binding to smooth muscle in tissue sections. Results were expressed as a titer, which was the highest dilution of patient sera giving a positive result. A titer greater than 1:20 was considered positive. With the immunofluorescent assay, many weakly positive ASMA were observed. Recently, an enzyme immunoassay using purified F-actin filaments has become available, which has better specificity. 

Interpretation

Current Method

New Method

Negative

<1:20

<20 Units

Weak Positive

20 – 30

Positive

>/=1:20

>30 Units

Specimen requirement is one red top or SST tube of blood.

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